1. Field of the Invention
The present invention relates to a technique for reducing the ohmic resistance of an ohmic electrode having a recessed structure and formed in a semiconductor device. The present invention may be applied to the source electrode and drain electrode of a High Electron Mobility Transistor (HEMT), for example.
2. Description of Related Art
In the prior art, a HEMT is known as a type of Field Effect Transistor (FET). A feature of a HEMT is that a current path is formed by a two-dimensional electron gas layer generated on the interface between two types of semiconductor film having different band gaps.
Typically, a HEMT comprises a channel-forming layer formed on a substrate and a Schottky layer formed on the channel-forming layer. Films having different band gaps are used as the channel-forming layer and Schottky layer. For example, a GaN film may be used as the channel-forming layer, and an AlGaN film may be used as the Schottky layer. The two-dimensional electron gas layer is formed on the interface between the channel-forming layer and the Schottky layer.
A cap layer is formed on the surface of the Schottky layer. A source electrode, a drain electrode, and a gate electrode are disposed on the cap layer. The source electrode and drain electrode are ohmic electrodes.
When a potential is applied to the gate electrode, a depletion layer is formed in the two-dimensional electron gas layer. A current flowing between the source electrode and the drain electrode is controlled by the depletion layer. In the two-dimensional electron gas layer, electron mobility is much greater than that of a normal semiconductor. Therefore, the HEMT operates at a higher speed than a typical FET.
To improve the characteristics of the HEMT, contact resistance between the ohmic electrodes and the two-dimensional electron gas layer is preferably reduced.
Ohmic contact resistance between a metal electrode and a semiconductor maybe calculated using the following Equation (1). This calculation method is disclosed in “3. Ultra High Speed Compound Semiconductor Devices”, edited by Masamichi Omori and supervised by Takuo Sugano, Baifukan, 6.2 Electrode Formation Technology (p. 196-202).Rc=((ρc×ρ□)0.5/W)×(1−e−L/Le)−1  (1)
In Equation (1), Rc is ohmic contact resistance (ohm×mm), ρc is contact resistivity (ohm×cm2), ρ□ is sheet resistance (ohm/sq), W is the gate width, and therefore the ohmic electrode width, and L is the ohmic electrode length. Further, Le is the ohmic electrode length when the current flowing into the ohmic electrodes is 1/e of the entire current.
It is usually believed that if L is 3×Le or more, the ohmic contact resistance can be sufficiently reduced. When L=3×Le, 1−e−L/Le=0.95. Hence, when L is sufficiently longer than Le, the following Equation (2) is approximately established.Rc=(ρc×ρ□)0.5/W)  (2)
The value of Le varies according to the sheet resistance and electron mobility of the two-dimensional electron gas layer. For example, when the sheet resistance is 400 Ohm and the electron mobility is 4000 cm2/Vs, Le=1.5 μm. Le increases as the electron mobility of the two-dimensional electron gas layer decreases, and therefore the length L of the ohmic electrodes must be increased. For example, when the channel-forming layer is formed from GaN and the Schottky layer is formed from AlGaN, the electron mobility is approximately 1500 cm2/Vs. Hence, in a HEMT using a hetero-junction of AlGaN and GaN, the source electrode and drain electrode must be made extremely long.
As another technique for reducing ohmic contact resistance between a metal electrode and a semiconductor, a method of alloying the contact surfaces is known. However, when semiconductors having a large energy gap are used, the energy required for alloying is extremely large, and therefore the alloying is difficult. In a HEMT using a hetero-junction of AlGaN and GaN, alloying is essentially impossible.
As a third technique for reducing ohmic contact resistance between a metal electrode and a semiconductor, an ohmic recess structure is known. In an ohmic recess structure, an ohmic electrode is buried in a recess formed in the surface of the semiconductor layer. For example, K. Kaifu et al. 2005 The Electrochemical Society, “AlGaN/GaN HEMTs with Recessed Ohmic Electrodes on Si Substrates”, ECS Transactions Vol. 1, No. 2, pp. 259-265 is known as a document which discloses an ohmic recess structure.
K. Kaifu et al. disclose a HEMT employing an ohmic recess in FIG. 2 thereof. As shown in this drawing, an ohmic electrode is buried in a recess formed in a GaN thin film by etching. K. Kaifu et al. reduce ohmic resistance by causing the two-dimensional electron gas layer to contact the bottom surface of the ohmic electrode directly.
However, according to an investigation conducted by the present inventor, the electric resistance of an ohmic contact formed between a semiconductor film and an ohmic electrode cannot be reduced sufficiently with the technique disclosed in K. Kaifu et al. This technique is also disadvantaged in that variation in the ohmic resistance value caused by manufacturing irregularities is large.